Early changes in AD brain include loss of synapses. AB, considered to have a central role in the pathogenesis of AD, bind to dendritic spines and cause synaptic dysfunction. However, the mechanisms responsible for AB-induced synaptic dysfunction and spine loss are not firmly established. Notably, synaptic terminals have abundant mitochondria which play an indispensable role at these sites. Along this line, mitochondrial dysfunction is an early prominent feature of AD neurons. Mitochondria are dynamic organelles that undergo continual fission and fusion events which are regulated by a machinery involving large dynamin-related GTPase that exert opposing effects, e.g., dynamin-like protein 1 (DLP1) and Fis1 for fission, and mitofusins (Mfn1 and Mfn2C) and OPA1 for fusion. These mitochondria fission and fusion proteins control not only mitochondrial number and morphology but also mitochondrial distribution and function. Indeed, defects in the mitochondrial fission/fusion balance and thus, the morphology and distribution have the potential to cause localized energy and calcium imbalance, which is especially damaging to polarized cells such as neurons, resulting in cellular dysfunction and death. Our preliminary studies suggest that the normally strict regulation of mitochondria morphology and distribution is impaired in AD neurons and fibroblasts which may be caused by differential expression of mitochondrial fission/fusion proteins induced by AB. Our central hypothesis is that AB induces mitochondrial dysfunction and synaptic abnormalities via its toxic effect on mitochondrial fission/fusion. Four aims will be pursued: Aim1) ADDLs induce mitochondrial dysfunction and synaptic abnormalities via its toxic effect on mitochondrial fission/fusion in vitro; Aim2) To Explore the Mechanisms of ADDL-induced DLP1 Reduction; Aim3) mutant PS1 causes mitochondrial abnormalities and neuronal dysfunction at least in part through its interaction with DLP1 and impaired balance in mitochondrial fission/fusion; Aim 4) DLP1 reduction underlies mitochondrial abnormalities and synaptic loss in vivo. PUBLIC HEALTH RELEVANCE: AB-caused synaptic dysfunction and spine loss is an early change and the most robust correlate of AD- associated cognitive deficits, however the underlying mechanism is not firmly established. It is known that mitochondria play an indispensable role in synaptic terminals and the balance of mitochondrial fission/fusion is critical for mitochondrial distribution and function. Our preliminary studies suggest the potential involvement of an impaired balance of mitochondrial fission/fusion in the pathogenesis of AD, in this application, we propose to investigate whether AB cause synaptic dysfunction and mitochondrial abnormalities via its toxic effect on the balance of mitochondrial fission and fusion.